Gallium nitride sintered body or gallium nitride molded article, and method for producing same

ABSTRACT

The present invention provides a gallium nitride sintered body and a gallium nitride molded article which have high density and low oxygen content without using a special apparatus. According to the first embodiment, a gallium nitride sintered body, which is characterized by having density of 2.5 g/cm 3  to less than 5.0 g/cm 3  and an intensity ratio of the gallium oxide peak of the (002) plane to the gallium nitride peak of the (002) plane of less than 3%, which is determined by X-ray diffraction analysis, can be obtained. According to the second embodiment, a metal gallium-impregnated gallium nitride molded article, which is characterized by comprising a gallium nitride phase and a metal gallium phase that exist as separate phases and having a molar ratio, Ga/(Ga+N), of 55% to 80%, can be obtained.

The present application is a Divisional Application of U.S. applicationSer. No. 13/992,486, filed on Jun. 7, 2013, which is a U.S. NationalStage Application of International Patent Application No.PCT/JP2011/079570 filed on Dec. 20, 2011, which claims priority toJapanese Application No. 2010-284631 filed on Dec. 21, 2010 and JapaneseApplication No. 2010-283165 filed on Dec. 20, 2010. The disclosures ofU.S. application Ser. No. 13/992,486 and International PatentApplication No. PCT/JP2011/079570 are incorporated by reference hereinin their entireties.

DESCRIPTION Technical Field

The present invention relates to a gallium nitride sintered body, agallium nitride molded article and methods of producing the same.

Background Art

Gallium nitride has been drawing attention as a luminescent layer of ablue light emitting diode (LED) and a material of a blue laser diode(LD). In recent years, gallium nitride has been used in the form of athin film or a substrate in a variety of applications such as white LEDsand blue LDs and has also drawn attention as a prospective material tobe used in applications such as power devices. At present a galliumnitride thin film is commonly produced by a metal organic chemical vapordeposition (MOCVD) method. In the MOCVD method, a vapor of a material isincorporated in a carrier gas and transported to the substrate surfacewhere the material is decomposed by a reaction with the heated substrateto allow crystal growth.

Examples of a method of preparing a thin film other than the MOCVDmethod include a sputtering method. In a sputtering method, a cationsuch as Ar ion is physically collided with a target provided on cathodeto allow the material constituting the target to be emitted by thecollision energy to deposit a film having substantially the samecomposition as the target material onto a substrate provided on theopposite side. Examples of such sputtering method include direct-currentsputtering method (DC sputtering method) and radio-frequency sputteringmethod (RF sputtering method).

Conventionally, a metal gallium target has been used in a method ofpreparing a gallium nitride thin film by sputtering (see, for example,Patent Document 1). However, in such cases where a metal gallium targetis used, the target may be melted because the melting point of metalgallium is about 29.8° C.; therefore, it is necessary that an expensivecooling device is installed and the film formation is performed at a lowpower in order to prevent the target from melting, and there is aproblem of reduced productivity.

Further, there has been proposed a gallium nitride thin film prepared byusing a sputtering target containing gallium nitride as a main component(see, for example, Patent Document 2); however, the density and physicalproperties of such gallium nitride target have not been examined. Inaddition, a system in which Tb or the like is added has been prepared asa sputtering target; however, no examination has been made on a systemconsisting of only gallium nitride or nitrogen and gallium.

Moreover, there has been proposed a high-density sintered body ofgallium nitride (see, for example, Patent Document 3). Such a sinteredbody is densified at a pressure of 58 Kbar (5.8 GPa); however, anapparatus required for applying such a pressure is very expensive andis, therefore, not suitable for preparing a large-sized sintered body.

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Unexamined Patent Publication Hei No.11-172424

[Patent Document 2] Japanese Unexamined Patent Publication Hei No.01-301850

[Patent Document 3] Japanese Unexamined Patent Publication No.2005-508822

NON-PATENT DOCUMENTS

[Non-patent Document 1] Transactions of The Materials Research Societyof Japan 29[6], 2781-2784 (2004)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Gallium nitride which may be used as a sputtering target or the like hasthe following problems.

The first problem is that a large-scale apparatus is required forproducing a high-density sintered body of gallium nitride. Galliumnitride is decomposed into gallium and nitride at about 1000° C. undernormal pressure; therefore, sintering does not proceed. In addition,gallium nitride has a low diffusion coefficient and the density thereofis hardly improved by ordinary sintering even at about 1000° C. In viewof this, a method for improving the density of a sintered body withoutrequiring a large-scale apparatus has been investigated and, forexample, a hot-pressing method has been employed (see, for example,Non-patent Document 1). However, the hot-pressing method can only yielda sintered body having density of about 48% (2.9 g/cm³) in terms ofrelative density, which is considerably low as a sputtering target.Since such a low-density sintered body contains a large number of pores,the electrical discharge at the time of sputtering is not stable. Inaddition, since such a sintered body has low thermal conductivity andheat is trapped in the surface side at the time of sputtering, crackingis likely to occur.

The second problem is the oxygen content. When gallium nitridecontaining a large amount of oxygen is used as a sputtering target, adesired gallium nitride thin film cannot be obtained with good purity,since a large amount of oxygen is also incorporated into the resultingthin film. In cases where the density of a gallium nitride target issimply improved as in the case of Patent Document 2, only a sinteredbody containing a large amount of oxygen can be obtained, since agallium nitride powder contains a certain amount of oxygen. Furthermore,when a compact sintered body is prepared, incorporation of an oxide intothe interior of the sintered body makes it difficult to remove oxygen.

The third problem is the difficulty of attaining a high film formationrate. Since a non-doped gallium nitride target has high electricalresistance and thus hardly conducts electricity, only a radio-frequency(RF) sputtering method can be used and a direct-current (DC) sputteringmethod, which is capable of forming a film at a high rate, cannot beused.

An object of the present invention is to provide a gallium nitridesintered body and a gallium nitride molded article which can solve theabove-described problems.

Means for Solving the Problems

A first embodiment of the present invention is directed to a galliumnitride sintered body which is characterized by having density of 2.5g/cm³ to less than 5.0 g/cm³ and an intensity ratio of the gallium oxidepeak of the (002) plane to the gallium nitride peak of the (002) planeof less than 3%, which is determined by a powder X-ray diffractionanalysis.

The gallium nitride sintered body according to the first embodiment ischaracterized by having an intensity ratio of the gallium oxide peak ofthe (002) plane to the gallium nitride peak of the (002) plane, which isdetermined by a powder X-ray diffraction analysis, of less than 3%. Theintensity ratio of the peaks can be determined by the powder X-raydiffraction analysis, based on the intensity ratio of the galliumnitride peak of the (002) plane and the gallium oxide peak of the (002)plane.

Conventionally, a gallium nitride sintered body may contain galliumoxide. When such a gallium oxide-containing sputtering target is used,it is difficult to obtain a gallium nitride thin film having excellentpurity, since a large amount of oxygen is also incorporated into theresulting sputtered thin film.

There may be a method of reducing the amount of gallium oxide containedin a gallium nitride powder used as a starting material in order toreduce the amount of gallium oxide in a gallium nitride sintered body.However, an oxide phase such as gallium oxide may exist in such agallium nitride powder as a material in such an amount that cannot bedetected by a powder X-ray diffraction. When a gallium nitride sinteredbody is produced by sintering such a gallium nitride powder and measuredby, for example, a powder X-ray diffraction, the resulting galliumnitride sintered body may contain a gallium oxide phase in such anamount that a gallium oxide peak can be detected. This is mainlyattributable to that the surface layer of gallium nitride powder iseasily oxidized. Further, even if a powder having a low oxygen contentwas prepared at the time of synthesizing a gallium nitride powder, it isdifficult to maintain a low oxygen content in the process between thepreparation of a gallium nitride powder and the preparation of a galliumnitride sintered body, since such a powder is likely to be affected byvarious ambient environments including storage and sintering conditions.

In view of the above, the present inventors intensively studied in orderto obtain a gallium nitride sintered body having a low oxygen contentand discovered that, as the first embodiment of the present invention, agallium nitride sintered body in which the intensity ratio of thegallium oxide peak to the gallium nitride peak in the (002) plane, whichis determined by X-ray diffraction analysis, is less than 3% can beobtained by heat-treating a gallium nitride sintered body in anammonia-containing atmosphere, and thus nitriding portions in thegallium oxide state in the resulting sintered body. The intensity ratioof the gallium oxide peak to the gallium nitride peak in the (002) planeis also preferably not higher than 1%.

In the first embodiment, the gallium nitride sintered body has an oxygencontent of preferably not higher than 11 atm %, more preferably nothigher than 5 atm %. The oxygen content of the sintered body can bemeasured using an oxygen-nitrogen analyzer.

In this embodiment, the gallium nitride sintered body has density of 2.5g/cm³ to less than 5.0 g/cm³, preferably 3.0 g/cm³ to 4.5 g/cm³. Theterm “density” of the gallium nitride sintered body refers to thedensity including open pores, and represents either a value calculatedfrom the weight and the volume which is determined from the shape of thesintered body or a bulk density measured in accordance with JIS R1634(Test methods for density and apparent porosity of fine ceramics). Thegallium nitride sintered body is required to have a desired strength tobe used as a sputtering target, and it is also preferred that thesintered body has density of not less than 2.5 g/cm³ in order to attainsuch a strength. It is also preferred that the gallium nitride sinteredbody has density of less than 5.0 g/cm³ in order to nitride the entiresintered body including the interior thereof by allowing an ammoniaatmosphere to permeate into the pores inside the sintered body, at thetime of subjecting the sintered body to a nitridation treatment in theammonia atmosphere to reduce the amount of gallium oxide.

The pores existing in the interior of the gallium nitride sintered bodyare classified into closed pores and open pores. A closed pore is anisolated pore existing in the interior of the sintered body, while anopen pore is a pore connected to the outer surface. The pores can bemeasured in accordance with JIS R1634 using the following formulas,wherein ρ₁ represents the density of measurement solvent; W₁ representsa dry weight; W₂ represents the weight-in-water; W₃ represents thewater-saturated weight; and ρ represents the true density of galliumnitride (6.087 g/cm³):Open pore volume: V _(o)=(W ₃ −W ₁)/ρ₁Closed pore volume: V _(c)=(W ₁ −W ₂)/ρ₁ −W ₁/ρThe ratio of open pores relative to all pores=V ₀/(V ₀ +V _(c))

In the gallium nitride sintered body of this embodiment, it ispreferable that not less than 70% by volume of the voids (pores)existing therein are open pores. This is because a greater number ofopen pores are more advantageous for achieving the reduction of theentire sintered body, since a reducing substance permeates through theopen pores during the reduction treatment of the sintered body. Morepreferably, the ratio of the open pores (open porosity) is not less than90% by volume.

A method of producing the gallium nitride sintered body according to thefirst embodiment is described hereinafter.

In this embodiment, as a result of detailed examination of therelationships between various physical properties of material powder,such as specific surface area (BET), untamped bulk density, and reposeangle, and the strength of the resulting sintered body, it wasdiscovered that incorporation of oxygen as an impurity can be reducedand a high-strength sintered body can be obtained, by controlling theabove various physical properties of gallium nitride powder. The term“untamped bulk density” used herein refers to a value which is obtainedby filling a container of a certain volume with a powder withoutapplying any load such as vibration and dividing the weight of thefilled powder by the volume of the container.

That is, in this embodiment, it is also preferred that the galliumnitride powder used as a starting material has a specific surface area(BET) of 0.4 m²/g to 15 m²/g. It is also preferred that the specificsurface area (BET) is not higher than 15 m²/g in order to reduceincorporation of oxygen as an impurity to obtain a highly crystallinefilm by sputtering. It is also preferred that the specific surface area(BET) is not less than 0.4 m²/g in order to improve the sinteringproperties of the gallium nitride powder to obtain a sintered bodyhaving sufficient strength.

It is also preferred that the gallium nitride powder has untamped bulkdensity of not less than 0.4 g/cm³. It is also preferred that theuntamped bulk density is not less than 0.4 g/cm³ in order to facilitatethe sintering of the gallium nitride to obtain a sintered body havingsufficient strength as a sputtering target.

It is also preferred that the gallium nitride powder has a repose angleof not larger than 40°. It is also preferred that the repose angle isnot larger than 40° in order to facilitate the sintering of the galliumnitride sintered body to obtain a sintered body having sufficientstrength as a sputtering target.

In this embodiment, it is also preferred that the gallium nitride powderto be used as a starting material contains impurities in the smallestamount possible in order to obtain a sputtering film having highcrystallinity or attain a desired change in the semiconductingproperties of the sputtering film by an addition of an element.Preferably, the gallium nitride powder is granulated.

Further, in this embodiment, a gallium nitride powder prepared bysubjecting a gallium oxide powder to a nitridation treatment in anammonia atmosphere at a temperature of 1000° C. to 1100° C. may also beemployed as the gallium nitride powder used as a starting material. Thegallium oxide powder can be sufficiently nitrided by setting thenitridation temperature at 1000° C. or higher. A gallium nitride powderhaving a desired specific surface area can be obtained by setting thenitridation temperature at not higher than 1100° C.

A variety of methods such as a pressureless sintering and a pressuresintering can be employed as the sintering method. An atmosphere whichhas a low oxygen concentration or contains no oxygen, such as a vacuumsintering atmosphere or an inert atmosphere, such as nitrogen and argon,can be employed as the sintering atmosphere in order to preventoxidation of the gallium nitride. The sintering can be performed by avariety of sintering methods. Preferably, a hot-pressing (HP) method ora hot isostatic pressing (HIP) method is employed.

In the hot-pressing method, sintering of a powder sample is facilitatedby applying heat thereto under an increased pressure, substancediffusion during sintering can be assisted by uniaxial pressingperformed at the time of heating, and the sintering of a material whichhas a low diffusion coefficient and is thus not easily sintered can befacilitated. The density of the resulting sintered body can be improvedas compared to conventional ones by sintering the above-describedgallium nitride powder using such a sintering method.

The hot isostatic pressing method is a method in which a sample can beheated while applying thereto pressure isostatically. The hot isostaticpressing method has an effect of assisting substance diffusion duringsintering in the same manner as the hot-pressing method and ischaracterized by being capable of yielding a high-density sintered bodybecause sintering can be performed under a higher pressure than in thehot-pressing method.

In this embodiment, the sintering temperature is preferably 900° C. to1050° C. in cases where a hot-pressing method is employed. The sinteringtemperature is also preferably not lower than 900° C. in order tofacilitate the sintering of gallium nitride, and the sinteringtemperature is also preferably not higher than 1050° C. in order toprevent the decomposition of gallium nitride into nitrogen and metalgallium. It is also preferred that the pressure during sintering iscontrolled at not less than 10 MPa in order to improve the density ofthe resulting sintered body. It is also preferred that the pressureduring sintering is not higher than 100 MPa from the standpoint ofpreventing damage of a carbon-made die commonly used in hot-pressing.

In cases where a hot-isostatic pressing method is employed, it is alsopreferred that the sintering temperature is 900° C. to 1050° C. in thesame manner as in the case of hot-pressing method and the pressureduring sintering is 10 MPa to 300 MPa. The pressure is also preferablynot less than 10 MPa in order to improve the density of the resultingsintered body. Moreover, the sintered body also has preferably a largeopen porosity in order to obtain a gallium nitride sintered body havinga low oxygen content by nitriding the interior of the sintered body inthe subsequent nitridation step of the sintered body. The pressureduring sintering is also preferably not higher than 300 MPa in order toobtain an open porosity of preferably not less than 70% by volume withrespect to the total volume of the voids consisting of open pores andclosed pores.

The thus obtained sintered body may also be processed into a prescribeddimension in accordance with the application thereof such as asputtering target. The processing method is not particularly limitedand, for example, a surface grinding method, a rotary grinding method,or a cylindrical grinding method can be employed.

In this embodiment, a gallium nitride sintered body produced by theabove-described method contains gallium oxide; therefore, the sinteredbody is subjected to nitridation in order to nitride the gallium oxidecontained therein. The nitridation of the sintered body can be performedby any arbitrary method capable of nitriding the gallium oxide containedin the sintered body. It is also preferred that the sintered body isheat-treated in an ammonia-containing atmosphere in order to ensurenitridation of the gallium oxide. An apparatus for performing thetreatment in an ammonia atmosphere may either be a closed system or aflow system. The apparatus is also preferably a flow system from thestandpoints of the safety and maintenance of reactivity. The amount ofammonia in the atmosphere appropriate for performing the treatment isvariable depending on the mass of the loaded gallium nitride and thepeak intensity ratio of gallium oxide to gallium nitride in the sinteredbody. It is sufficient as long as ammonia gas is flowed through in anamount comparable to the amount of the gallium nitride sintered body. Itis also preferred that a larger amount of ammonia is flowed through,since the reactivity between ammonia and gallium oxide is furtherimproved. In addition, the treatment temperature is also preferably inthe range from 800° C., at which gallium oxide starts to be nitrided, to1200° C. It is particularly preferred that the treatment temperature is900° C. to 1200° C. from the standpoint of the reactivity betweenammonia and gallium oxide. Gallium oxide existing in gallium nitride canbe nitrided and a sintered body having a single gallium nitride phasecan be obtained by controlling the treatment temperature in theabove-described range.

The sintered body prior to being subjected to the nitridation treatmenthas density of preferably less than 5.0 g/cm³, more preferably less than4.5 g/cm³, in order to achieve nitridation of the entire sintered bodyincluding the interior thereof.

The sintered body prior to being subjected to the nitridation treatmenthas an open porosity of also preferably not less than 70% by volume,more preferably not less than 90% by volume with respect to the totalvolume of the voids. In a reduction treatment of the sintered body whichis performed using a reducing substance such as ammonia, the reducingsubstance permeates through the open pores; therefore, the more openpores exist in the sintered body, the more easily the entire sinteredbody is reduced. In addition, at the time of nitriding gallium oxide,distortion due to changes in the volume and the crystal of the galliumoxide caused by the nitridation can be absorbed by the open pores of thesintered body.

A second embodiment of the present invention is directed to a metalgallium-impregnated gallium nitride molded article which ischaracterized by comprising gallium nitride and metal gallium asseparate phases and having a molar ratio, Ga/(Ga+N), of 55% to 80% inthe entirety of the molded article. The molar ratio, Ga/(Ga+N), is avalue which represents the molar ratio of Ga and (Ga+N) in terms ofpercentage.

The structure of the metal gallium-impregnated gallium nitride moldedarticle according to this embodiment is described hereinafter referringto FIG. 3. In this embodiment, the term “molded article” encompassesmolded products that are obtained by hardening powder by a variety ofmethods such as a molding and a sintering. In cases where the metalgallium-impregnated gallium nitride molded article is used as asputtering target, it is also preferred that the molded article containsa sintered body having prescribed density, since the stronger the targetis, the less likely cracking occurs in the target during use; however,any molded article having prescribed density as a whole may be used.

A metal gallium-impregnated gallium nitride molded article 11 shown inFIG. 3 is characterized by comprising a gallium nitride phase 12 and ametal gallium phase 13 as separate phases and having a molar ratio,Ga/(Ga+N), of 55% to 80% in the entirety of the molded article. Theexistence of the gallium nitride and the metal gallium in the moldedarticle as separate phases can be confirmed by identifying the structureby a X-ray diffraction analysis. It can also be confirmed whether theanalyzed spot is of a single gallium phase or a phase containingnitrogen and gallium by using an analytical method such as an EPMA(electron probe microanalyzer) or an EDS (energy dispersive X-raydiffraction spectrometer). The metal gallium can also be detected andquantified by separating the metal gallium by, for example,heat-treating the molded article at about 150° C. to 250° C. to allowthe metal gallium to be melted and exuded from the molded article.

The molar ratio of Ga/(Ga+N) in the entirety of the molded article canbe determined by performing, for example, an elementary analysis usingan EPMA, an XPS (X-ray photoelectron spectrometer) or the like.Alternatively, the molar ratio of Ga/(Ga+N) can be determined by, forexample, thermally decomposing the gallium nitride molded article tomeasure the amount of nitrogen by thermal conductivity method (using,for example, an oxygen-nitrogen analyzer manufactured by LECOCorporation) and then measuring the gallium content in the moldedarticle by performing an elementary analysis such as ICP emissionspectroscopy.

In this embodiment, the thermal conductivity of the metalgallium-impregnated gallium nitride molded article can be improved byincorporating therein a prescribed amount of metal gallium. The molarratio of Ga/(Ga+N) is also preferably not lower than 55% in order toattain a desired effect of improving the electrical conductivity of themetal gallium-impregnated gallium nitride molded article. Further, incases where the metal gallium-impregnated gallium nitride molded articleis used as a sputtering target, the molar ratio of Ga/(Ga+N) is alsopreferably not higher than 80% in order to achieve stable filmformation. The metal gallium may be melted and markedly exuded from thetarget surface to affect the film formation when the ratio of metalgallium is excessively high. More preferably, the molar ratio ofGa/(Ga+N) is 60% to 80% in order to attain more stable electricalconductivity and thermal conductivity.

In this embodiment, it is also preferred that not less than 30% byvolume of the total volume of the voids contained in the metalgallium-impregnated gallium nitride molded article is filled with themetal gallium. The term “void” used herein refers to a pore contained inthe above-described molded article and the pore may be open or closed.

The amount of the metal gallium impregnated into the metalgallium-impregnated gallium nitride molded article is determined by, forexample, performing a mapping analysis for gallium and nitrogen that arecontained in the molded article by using an EDS, an EPMA or the like.The metal gallium-impregnated gallium nitride molded article accordingto this embodiment contains the gallium nitride having a molar ratio(gallium:nitrogen) of about 50:50 and the metal gallium; therefore, byperforming a mapping analysis, the amount of the part of the metalgallium (C) can be determined by the following formula, C=A−B, takingthe part where nitrogen was detected excluding the background as galliumnitride (B) and the part where gallium was detected excluding thebackground as total gallium (A). A void (D) is a part where neithergallium nor nitrogen is detected or a part observed as a pore under aSEM (scanning electron microscope). The volume ratio of metal gallium(X) with respect to voids (D) is a value which is calculated using thefollowing formula, X=(A−B)/(A−B+D), and indicated in percentage.

In the gallium nitride molded article according to this embodiment inwhich not less than 30% of the total volume of the voids is filled withthe metal gallium, the metal gallium can exist continuously, since themetal gallium is likely to impregnate into the interior of the galliumnitride molded article. In this manner, since the metal gallium coversthe surface layer of the gallium nitride even in fine pores, it becomeseasier to prevent the oxidation of the surface layer of the galliumnitride. In addition, the density and the electrical conductivity of themolded article can be improved by allowing the metal gallium tocontinuously exist into the interior of the molded article, and stableelectrical discharge is therefore likely to be generated when the moldedarticle is used as a sputtering target for preparing a gallium nitridethin film.

The upper limit of the amount of the metal gallium contained in thevoids of the metal gallium-impregnated gallium nitride molded article isnot particularly restricted; however, it is also preferred that thevolume of the metal gallium is not greater than 90% with respect to thetotal volume of the voids, since the metal gallium may protrude due tothe difference in the thermal expansions between the metal gallium andthe gallium nitride sintered body caused by a slight increase in thetemperature of the whole target at the time of sputtering. In thismanner, occurrence of a defect (pinhole) can be prevented in theresulting thin film and a high yield can be stably maintained.

In this embodiment, it is also preferred that the metalgallium-impregnated gallium nitride molded article has density of 3.20g/cm³ to less than 6.05 g/cm³. Since the molded article contains voidsthat are open to the outside, the “density” of the metalgallium-impregnated gallium nitride molded article refers to the densitycalculated from the mass and the apparent volume which is estimated fromthe shape of the molded article. In cases where the metalgallium-impregnated gallium nitride molded article is used as asputtering target, the metal gallium-impregnated gallium nitride moldedarticle has density of preferably not less than 3.20 g/cm³, morepreferably not less than 4.00 g/cm³, in order to prevent occurrence ofan abnormal electrical discharge at the time of sputtering. Meanwhile,excessively high density may prevent the metal gallium from impregnatinginto the interior of the molded article, which may cause an abnormalelectrical discharge. Preferably, the metal gallium-impregnated galliumnitride molded article has density of less than 6.05 g/cm³ in order toobtain a molded article whose interior is impregnated with the metalgallium.

In this embodiment, it is also preferred that the metalgallium-impregnated gallium nitride molded article has resistance of nothigher than 1 Ω·cm. An electrical discharge is hardly generated by DCsputtering and only RF sputtering can produce an electrical dischargewhen the resistance of a gallium nitride target is high; however, whenthe resistance is 1 Ω·cm or less, the film productivity is furtherimproved and the production equipment becomes less expensive, since afilm is easily formed by DC sputtering as well. It is more preferredthat the metal gallium-impregnated gallium nitride molded article hasresistance of not higher than 0.1 Ω·cm in order to allow an electricaldischarge to be more stably generated.

In this embodiment, the metal gallium-impregnated gallium nitride moldedarticle has higher thermal conductivity as compared to a gallium nitridemolded article prior to being impregnated with metal gallium. The morethe metal gallium is contained in the molded article, the higher becomesthe thermal conductivity of the whole molded article, since the thermalconductivity of the gallium nitride is lower than that of the metalgallium. Since the metal gallium-impregnated gallium nitride moldedarticle has high thermal conductivity, when it is used as a sputteringtarget, cracking caused by thermal stress is not likely to occur evenwhen a high power is applied. Accordingly, a thin film produced by asputtering method has a small number of defects such as pinhole and thetarget can be used for a prolonged period with good yield.

Since the gallium nitride molded article prior to being impregnated withmetal gallium contains voids in a large volume, the thermal conductivitythereof is 3 W/mK or less; however, the metal gallium-impregnatedgallium nitride molded article according to this embodiment can havethermal conductivity of 10 W/mK or higher.

In this embodiment, the amount of oxygen contained in the metalgallium-impregnated gallium nitride molded article is also preferablynot higher than 11 atm %. The amount of oxygen contained in the moldedarticle can be measured by, for example, an oxygen-nitrogen analyzer(manufactured by LECO Corporation). Alternatively, the amount of oxygencan also be calculated from the intensity ratio of the gallium oxidepeak of the (002) plane to the gallium nitride peak of the (002) plane,which is determined by measuring the molded article by a powder X-raydiffraction. It was discovered that the oxygen content is 11 atom % orless when the peak intensity ratio is less than 3%, as a result ofdetailed examination of the relationship between the intensity ratio ofthe gallium oxide peak to the gallium nitride peak in the (002) planeand the oxygen content.

Next, a method of producing the metal gallium-impregnated galliumnitride molded article according to this embodiment will be described indetail.

First, a gallium nitride molded article is produced using a galliumnitride powder. It is also preferred that the gallium nitride powder tobe used contains impurities in the smallest amount possible and has apurity of not less than 4 N. The method of making the gallium nitridemolded article is not particularly limited, and a variety of methodssuch as a method of molding a powder, a method of sintering a moldedproduct, a hot-pressing (HP) treatment method, and a hot isostaticpressing (HIP) treatment method can be employed. It is also preferred toemploy a method by which a molded article having a size of not smallerthan 100 mmϕ can be easily produced. It is also preferred to employ amethod by which a molded article having a high strength can be produced,since the strength of the gallium nitride molded article affects themechanical strength of the metal gallium-impregnated gallium nitridemolded article produced therefrom.

It is also preferred that the gallium nitride molded article prior tobeing impregnated with metal gallium has density of 2.0 g/cm³ to 5.0g/cm³. The gallium nitride molded article prior to being impregnatedwith the metal gallium has density of preferably not less than 2.0g/cm³, more preferably not less than 2.5 g/cm³, still more preferablynot less than 3.0 g/cm³, in order to prevent the molded article frombeing cracked when impregnated with the metal gallium and to provide theresulting metal gallium-impregnated gallium nitride molded article withsufficient strength when used as a sputtering target. The galliumnitride molded article prior to being impregnated with metal gallium hasdensity of preferably not higher than 5.0 g/cm³, more preferably nothigher than 4.5 g/cm³ in order to obtain a molded article having desiredresistivity by utilizing the pores that are open to the interior of themolded article to allow the metal gallium to exist continuously in theinterior of the molded article.

In this embodiment, the gallium nitride molded article obtained in theabove-described manner is impregnated with metal gallium. As a method ofimpregnating the metal gallium into the above-described gallium nitridemolded article, for example, but not limited to, the below-describedmethod is convenient and practical. The gallium nitride molded articleprior to being impregnated with metal gallium may be in an arbitraryform by, for example, being processed into a variety of shapes oradhered to a backing plate. When such gallium nitride molded article isimpregnated with the metal gallium, it is difficult to achieveimpregnation by mere immersion of a gallium nitride in metal galliumsolution, since the wettability between the gallium nitride and themetal gallium is not high.

The method of impregnating the metal gallium into a gallium nitridemolded article will now be described referring to FIGS. 4 to 6. Theimpregnation method comprises the following steps: (1) the step ofremoving gas contained in a gallium nitride molded article (FIG. 4); (2)the step of packaging metal gallium along with the gallium nitridemolded article under vacuum (FIG. 5); and (3) the step of impregnatingthe metal gallium in a liquid state into the gallium nitride moldedarticle with application of a prescribed pressure (FIG. 6). The metalgallium can be uniformly impregnated into the gallium nitride moldedarticle by going through these steps.

The lower the oxygen content in the metal gallium used as a startingmaterial is, the better it is. This is because the oxygen content of theresulting metal gallium-impregnated gallium nitride molded article canbe reduced as long as the oxygen content in the metal gallium is loweven when a large amount of oxygen is contained in the gallium nitridemolded article prior to being impregnated with metal gallium. The amountof oxygen in the metal gallium is preferably not higher than 1 atm %,more preferably not higher than 0.1 atm %. The metal gallium may also bemelted at a temperature of not lower than its melting point in a vacuumoven and subjected to a degassing treatment in order to reduce theamount of oxygen in the metal gallium.

Further, each step will now be described in detail.

First, the step (1) is described. For a gallium nitride molded article12 having voids 14 as shown in FIG. 4, gas contained in the voids 14 isremoved by vacuuming so as to allow the metal gallium to impregnate intothe interior. The vacuuming is performed to the molded article such thata residual gas pressure inside the molded article becomes preferably nothigher than 10000 Pa, more preferably not higher than 1000 Pa, stillmore preferably not higher than 100 Pa, in order to allow the metalgallium to almost completely impregnate into the molded article.

Next, the step (2) is described. Vacuuming is performed in a state wherethe gallium nitride molded article 12 and the metal gallium 15 areenclosed in a vacuum packaging bag 16 as shown in FIG. 5. The metalgallium 15 may be in a solid state or a liquid state under vacuumcondition. The position of the metal gallium 15 in the vacuum packagingbag 16 is not particularly restricted; however, it is also preferredthat the metal gallium 15 is uniformly arranged in the periphery of thegallium nitride molded article 12 as much as possible. The amount of themetal gallium loaded into the vacuum packaging bag 16 is also preferablynot less than 30% with respect to the volume of the voids in the galliumnitride molded article in order to allow the entire gallium nitridemolded article 12 to be uniformly impregnated with the metal gallium. Ata smaller amount, since the metal gallium becomes hard to impregnateinto the interior of the gallium nitride molded article 12, a metalgallium-impregnated gallium nitride molded article having a uniformstructure is difficult to be obtained.

The material of the vacuum packaging bag 16 is not particularly limited,and a commonly used bag, such as an aluminum-deposited bag, a nylon bag,or a polyethylene bag, may be employed.

Next, the step (3) is described. The metal gallium is allowed toimpregnate into the gallium nitride molded article by externallyapplying pressure to the vacuum packaging bag 16 as shown in FIG. 6. Incases where the metal gallium is in a solid state, it may be made into aliquid in advance by a heat treatment or the like. There is noparticular restriction on the method of applying pressure and, forexample, a uniaxial pressing method, a cold isostatic pressing (CIP)method, or a hot isostatic pressing (HIP) method can be employed;however, it is also preferred to employ a method by which pressure isexternally applied to the vacuum packaging bag 16 in an isotropicmanner, since it is also preferred that the metal gallium is impregnatedfrom various directions. It is required that the temperature at the timeof applying pressure is controlled to be higher than the solidificationtemperature of the metal gallium. After the application of pressure, theresulting metal gallium-impregnated gallium nitride molded article istaken out of the vacuum packaging bag 16 and the metal gallium remainingin the periphery of the molded article is removed.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle may also be processed into a prescribed dimension in accordancewith the application thereof such as a sputtering target. The processingmethod is not particularly limited and, for example, a surface grindingmethod, a rotary grinding method, or a cylindrical grinding method canbe employed.

The obtained metal gallium-impregnated gallium nitride molded articlemay also be subjected to a heat treatment. The metal gallium mayprotrude from the molded article to the outside due to the difference inthe thermal expansions of the gallium nitride and that of the metalgallium when the metal gallium-impregnated gallium nitride moldedarticle is heated at the time of sputtering or the like. The protrusionof the metal gallium can be prevented when the molded article is heatedat the time of sputtering or the like, by heat-treating the metalgallium-impregnated gallium nitride molded article at a prescribedtemperature to allow the metal gallium to protrude and removing theprotruded metal gallium in advance. In the heat treatment, it is alsopreferred that the amount of the metal gallium remaining in the moldedarticle is not less than 30% by volume with respect to the total volumeof the voids, since the electrical conductivity of the metalgallium-impregnated gallium nitride molded article may be reduced byexcessive protrusion of the metal gallium.

A third embodiment of the present invention is a combination of thefirst and second embodiments.

That is, the third embodiment is a metal gallium-impregnated galliumnitride molded article which comprises a gallium nitride sintered bodyand metal gallium, wherein the gallium nitride sintered body has densityof 2.5 g/cm³ to less than 5.0 g/cm³ and has a composition in which theintensity ratio of the gallium oxide peak to the gallium nitride peak inthe (002) plane is less than 3%, which is determined by a powder X-raydiffraction analysis, and wherein the metal gallium-impregnated galliumnitride molded article comprises the gallium nitride and the metalgallium as separate phases and has a molar ratio, Ga/(Ga+N), of 55% to80% in the entirety of the molded article.

The metal gallium-impregnated gallium nitride molded article accordingto the third embodiment of the present invention can be produced byusing the gallium nitride sintered body of the first embodiment andapplying the above-described production method of the second embodiment.

According to the third embodiment, a metal gallium-impregnated galliumnitride molded article which has properties of both the gallium nitridesintered body according to the first embodiment and the metalgallium-impregnated gallium nitride molded article according to thesecond embodiment can be obtained.

The gallium nitride sintered body or the metal gallium-impregnatedgallium nitride molded article according to the first to the thirdembodiments may also be fixed (bonded) onto a flat or cylindricalsupport medium using an adhesive such as a solder material as required.The material of the support medium is not particularly limited as longas it has high thermal conductivity and a strength capable of supportingthe molded article; however, the material is also preferably a metalsuch as Cu, SUS, or Ti, because of the high thermal conductivity andhigh strength thereof. As for the shape of the support medium, it isalso preferred that a flat support medium be used for a flat-shapedsintered body or molded article and a cylindrical support medium be usedfor a cylindrically-shaped sintered body or molded article. The adhesive(bonding material) used for adhering the sintered body or molded articleonto the support medium is not particularly limited as long as it has anadhesive strength sufficient for supporting the sintered body or moldedarticle onto the support medium. An electroconductive resin, a tin-basedsolder material, or an indium-based solder material can be employed assuch an adhesive. Thereamong, an indium solder having high electricalconductivity and thermal conductivity as well as a softness to be easilydeformed is also preferably employed. This is because when the galliumnitride sintered body or molded article is used as a sputtering target,the target surface can be efficiently cooled by using such an indiumsolder to adhere the gallium nitride sintered body or molded articleonto the support medium, and the indium solder can prevent cracking ofthe sintered body or molded article by absorbing a stress generated bythe difference of the thermal expansions between the sintered body ormolded article and the support medium.

In cases where an indium-based solder material is used, a barrier layersuch as tungsten, which has low reactivity with gallium, may also beformed between the sintered body or molded article and the soldermaterial in order to prevent a reaction between the metal galliumimpregnated into the gallium nitride sintered body or molded article andthe solder material. It is also preferred that such a barrier layer isformed uniformly over the entire interface between the sintered body ormolded article and the solder material. The method of forming such abarrier layer is not particularly limited, and the barrier layer can beformed by, for example, sputtering, vapor deposition, or coating.

Effects of the Invention

According to the first embodiment of the present invention, a galliumnitride sintered body having a low oxygen content can be obtained.

According to the second embodiment of the present invention, a galliumnitride molded article having low resistivity and large thermalconductivity can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction spectrum of the gallium nitridesintered body prepared in Example 1 of the first embodiment.

FIG. 2 shows an X-ray diffraction spectrum of the gallium nitridesintered body prepared in Comparative Example 1 of the first embodiment.

FIG. 3 is a schematic diagram showing a cross-section of a metalgallium-impregnated gallium nitride molded article according to thesecond embodiment.

FIG. 4 is a schematic diagram showing a cross-section of a galliumnitride molded article from which gas was removed, in the secondembodiment.

FIG. 5 is a schematic diagram showing a cross-section of a galliumnitride molded article and metal gallium that are packaged in a vacuumpackaging container, in the second embodiment.

FIG. 6 is a schematic diagram showing a cross-section of a galliumnitride molded article impregnated with metal gallium under a prescribedpressure.

FIG. 7 shows a powder X-ray diffraction spectrum of the metalgallium-impregnated gallium nitride molded article obtained in Example 1of the second embodiment.

EXAMPLES

The first embodiment of the present invention will now be described byway of examples thereof; however, this embodiment is not limitedthereto.

(Density)

The density of a sintered body was measured in accordance with themethod of measuring the bulk density prescribed in JIS R1634.

(Oxygen Content)

The oxygen content of a sintered body was measured using anoxygen/nitrogen analyzer (manufactured by LECO Corporation).

(Repose Angle)

The repose angle, which is a parameter of powder fluidity, was measuredusing a powder tester (Model PT-N, manufactured by Hosokawa MicronGroup).

(Specific Surface Area)

The specific surface area of a powder was measured using MicromeriticsTristar (manufactured by Shimadzu Corporation).

(Untamped Bulk Density)

The untamped bulk density of a powder was measured in accordance withJIS 22504.

First Embodiment—Example 1

A gallium nitride powder (100 g, purity of 4 N; manufactured by KojundoChemical Lab. Co., Ltd.) having a specific surface area (BET) of 14m²/g, untamped bulk density of 0.551 g/cm³, and a repose angle of 39°was loaded in a 102 mmϕ carbon-made die to perform hot-pressing. Thehot-pressing treatment was performed by heating the gallium nitridepowder at a rate of 200° C./h to a final temperature of 1000° C.,increasing the pressing pressure to 40 MPa when the temperature reaches1000° C., and maintaining the temperature and pressure for 2 hours.After the 2 hours of retention time, the resultant was cooled to about50° C. over a period of 5 hours and the die was taken out to remove agallium nitride sintered body. The thus obtained gallium nitridesintered body had density of 2.75 g/cm³, and the volume ratio of openpores with respect to the total volume of open pores and closed poreswas 98%. The gallium nitride sintered body was then processed into a76.2 mmϕ×2.0 mmt disk.

Then, 25.0 g of the thus processed gallium nitride sintered body wasloaded in a tube furnace. The sintered body was heated to 1000° C. at arate of 300° C./h and maintained at 1000° C. for 2 hours in an ammoniaatmosphere in which ammonia gas was flowed through at 200 ml/min, toperform a nitridation treatment of the sintered body. The galliumnitride sintered body was analyzed by a powder X-ray diffraction (XRD;RINT Ultima III, manufactured by Rigaku Corporation) after thenitridation treatment to obtain the X-ray diffraction spectrum shown inFIG. 1. There was observed no peak corresponding to gallium oxide in theX-ray diffraction spectrum shown in FIG. 1; therefore, it was found thatthe thus obtained gallium nitride sintered body contained either nogallium oxide or only a trace amount of gallium oxide below the lowerdetection limit. The density, open porosity, oxygen content, andpresence/absence of cracking of the obtained gallium nitride sinteredbody are shown in Table 2.

The obtained gallium nitride sintered body was bonded onto a backingplate made of Cu by using an indium solder as a bonding material toobtain a gallium nitride sputtering target.

First Embodiment—Example 2

The same gallium nitride powder as used in Example 1 (3 g, purity: 4 N)was loaded in a 20-mmϕ carbon-made die to perform hot-pressing. Thehot-pressing treatment was performed by heating the gallium nitridepowder at a rate of 200° C./h to a final temperature of 1000° C.,increasing the pressing pressure to 100 MPa when the temperature reaches1000° C., and maintaining the temperature and pressure for 2 hours.After the 2 hours of retention time, the resultant was cooled to about50° C. over a period of 5 hours and the die was taken out to remove agallium nitride sintered body. The gallium nitride sintered body wasthen processed into a 20.0 mmϕ×2.0 mmt disk.

Then, 2.5 g of the thus processed gallium nitride sintered body wasloaded in a tube furnace. The sintered body was heated to 900° C. at arate of 300° C./h and maintained at 900° C. for 2 hours in an ammoniaatmosphere in which ammonia gas was flowed through at 100 ml/min, toperform a nitridation treatment of the sintered body. The galliumnitride sintered body was analyzed by the XRD after the nitridationtreatment, and there was observed no peak corresponding to gallium oxidein the X-ray diffraction spectrum; therefore, it was found that the thusobtained gallium nitride sintered body contained either no gallium oxideor only a trace amount of gallium oxide below the lower detection limit.The density, open porosity, X-ray peak intensity ratio, oxygen content,and presence/absence of cracking of the obtained gallium nitridesintered body are shown in Table 2.

First Embodiment—Example 3

A gallium oxide powder (200 g, purity: 4 N, manufactured by Nippon RareMetal, Inc.) was loaded in a tube furnace. The gallium oxide powder washeated to 1050° C. at a rate of 600° C./h and maintained at 1050° C. for5 hours to be nitrided in an ammonia atmosphere in which ammonia gas wasflowed through at 400 ml/min, to obtain a gallium nitride powder. Aportion of this gallium nitride powder was collected, and its specificsurface area (BET), untamped bulk density, and repose angle weremeasured. The physical property values of the obtained gallium nitridepowder are shown in Table 1.

Then, 100 g of the thus obtained gallium nitride powder was loaded in a102 mmϕ carbon-made die to perform a hot press. A hot-pressing treatmentwas performed by heating the gallium nitride powder at a rate of 200°C./h to a final temperature of 1050° C., increasing the pressingpressure to 50 MPa when the temperature reaches 1050° C., andmaintaining the temperature and pressure for 2 hours. After the 2 hoursof retention time, the resultant was cooled to about 50° C. over aperiod of 5 hours and the die was taken out to remove a gallium nitridesintered body. The obtained sintered body was then processed into a 76.2mmϕ×2 mmt disk.

Then, 28.0 g of the thus processed gallium nitride sintered body wasloaded in a tube furnace. The sintered body was heated to 1050° C. at arate of 300° C./h and maintained at 1050° C. for 2 hours in an ammoniaatmosphere in which ammonia gas was flowed through at 200 ml/min, toperform a nitridation treatment of the sintered body.

The gallium nitride sintered body was analyzed by the XRD after thenitridation treatment, and there was observed no peak corresponding togallium oxide in the X-ray diffraction spectrum; therefore, it was foundthat the thus obtained gallium nitride sintered body contained either nogallium oxide or only a trace amount of gallium oxide below the lowerdetection limit. The density, open porosity, X-ray peak intensity ratio,oxygen content, and presence/absence of cracking of the obtained galliumnitride sintered body are shown in Table 2.

The gallium nitride sintered body was bonded onto a backing plate madeof Cu by using an indium solder as a bonding material to obtain agallium nitride sputtering target having no breakage or cracking.

First Embodiment—Example 4

The same gallium oxide powder as used in Example 3 (200 g, purity: 4 N)was loaded in a tube furnace. The gallium oxide powder was heated to1000° C. at a rate of 600° C./h and maintained at 1000° C. for 5 hoursto be nitrided in an ammonia atmosphere in which ammonia gas was flowedthrough at 400 ml/min, to obtain a gallium nitride powder. A portion ofthis gallium nitride powder was collected, and its specific surface area(BET), untamped bulk density, and repose angle were measured.

The physical property values of the obtained gallium nitride powder areshown in Table 1.

After subjecting the thus obtained gallium nitride powder to ahot-pressing treatment under the same conditions as in Example 3, anitridation treatment of the resulting gallium nitride sintered body wasperformed under the same conditions as in Example 3. The density, openporosity, X-ray peak intensity ratio, oxygen content, andpresence/absence of cracking of the obtained gallium nitride sinteredbody are shown in Table 2.

First Embodiment—Example 5

The same gallium oxide powder as used in Example 3 (200 g, purity: 4 N)was loaded in a tube furnace. The gallium oxide powder was heated to1,100° C. at a rate of 600° C./h and maintained at 1,100° C. for 5 hoursto be nitrided in an ammonia atmosphere in which ammonia gas was flowedthrough at 400 ml/min, to obtain a gallium nitride powder. A portion ofthis gallium nitride powder was collected, and its specific surface area(BET), untamped bulk density, and repose angle were measured. Thephysical property values of the gallium nitride powder are shown inTable 1.

After subjecting the thus obtained gallium nitride powder to ahot-pressing treatment under the same conditions as in Example 3, anitridation treatment of the resulting gallium nitride sintered body wasperformed under the same conditions as in Example 3. The density, openporosity, X-ray peak intensity ratio, oxygen content, andpresence/absence of cracking of the obtained gallium nitride sinteredbody are shown in Table 2.

First Embodiment—Example 6

A gallium nitride powder in an amount of 100 g, which was obtained inthe same manner as in Example 3, was loaded in a 102 mmϕ carbon-made dieto perform hot-pressing. The hot-pressing treatment was performed byheating the gallium nitride powder at a rate of 200° C./h to a finaltemperature of 1050° C., increasing the pressing pressure to 100 MPawhen the temperature reaches 1050° C., and maintaining the temperatureand pressure for 2 hours. After the 2 hours of retention time, theresultant was cooled to about 50° C. over a period of 5 hours and thedie was taken out to remove a gallium nitride sintered body. Theobtained sintered body was then processed into a 76.2 mmϕ×2 mmt disk.

Then, 37.8 g of the thus processed gallium nitride sintered body wasloaded in a tube furnace. The sintered body was heated to 1,050° C. at arate of 300° C./h and maintained at 1,050° C. for 2 hours in an ammoniaatmosphere in which ammonia gas was flowed through at 200 ml/min, toperform a nitridation treatment of the sintered body. The galliumnitride sintered body was analyzed by the XRD after the nitridationtreatment, and a peak corresponding to gallium oxide was not observed inthe X-ray diffraction spectrum; therefore, it was found that the thusobtained gallium nitride sintered body contained either no gallium oxideor only a trace amount of gallium oxide below the lower detection limit.The density, open porosity, X-ray peak intensity ratio, oxygen content,and presence/absence of cracking of the obtained gallium nitridesintered body are shown in Table 2.

The gallium nitride sintered body was bonded onto a backing plate madeof Cu by using an indium solder as a bonding material to obtain agallium nitride sputtering target having no breakage or cracking.

First Embodiment—Example 7

A gallium nitride powder in an amount of 100 g, which was obtained inthe same manner as in Example 3, was loaded in a 102 mmϕ die made of SUSto perform a hot isostatic pressing. The hot isostatic pressingtreatment was performed by heating the gallium nitride powder at a rateof 100° C./h to a final temperature of 1050° C., increasing the pressingpressure to 260 MPa when the temperature reaches 1050° C., andmaintaining the temperature and pressure for 1 hour. After the 2 hoursof retention time, the resultant was cooled to about 50° C. over aperiod of 8 hours and the die was taken out to remove a sintered body.The obtained sintered body was then processed into a 76.2 mmϕ×2 mmtdisk.

Then, 43.8 g of the thus processed gallium nitride sintered body wasloaded in a tube furnace. The sintered body was heated to 1050° C. at arate of 300° C./h and maintained at 1050° C. for 2 hours in an ammoniaatmosphere in which ammonia gas was flowed through at 200 ml/min, toperform a nitridation treatment of the sintered body. The galliumnitride sintered body was analyzed by XRD after the nitridationtreatment, and the intensity ratio of the gallium oxide peak to thegallium nitride peak in the (002) was determined to be 2.5% from theX-ray diffraction spectrum; therefore, it was found that the thusobtained gallium nitride sintered body contained only a minor amount ofgallium oxide. The density, open porosity, X-ray peak intensity ratio,oxygen content, and presence/absence of cracking of the obtainedsintered body are shown in Table 2.

The gallium nitride sintered body was bonded onto a backing plate madeof Cu by using an indium solder as a bonding material to obtain agallium nitride sputtering target having no breakage or cracking.

First Embodiment—Comparative Example 1

A gallium nitride sintered body was produced in the same manner as inExample 1 up to the point before the nitridation treatment. The obtainedsintered body was analyzed by the XRD without performing a nitridationtreatment, and the X-ray diffraction spectrum shown in FIG. 2 wasobtained and the intensity ratio of the gallium oxide peak to thegallium nitride peak in the (002) plane was determined to be 5.9%;therefore, it was found that the obtained gallium nitride sintered bodycontained a large amount of gallium oxide.

First Embodiment—Comparative Example 2

A gallium nitride powder was obtained by subjecting a gallium oxidepowder to a nitridation treatment in the same manner as in Example 3,except that the nitridation temperature was changed to 960° C. A portionof this gallium nitride powder was collected, and its specific surfacearea (BET) was measured. In addition, the untamped bulk density wasmeasured in accordance with JIS Z2504, and the repose angle was alsomeasured using a powder tester (Model PT-N, manufactured by HosokawaMicron Group). The physical property values of the obtained galliumnitride powder are shown in Table 1.

The thus obtained gallium nitride powder was subjected to ahot-pressing, nitridation, and bonding in the same manner as in Example3 to prepare a gallium nitride sintered body and a gallium nitridesputtering target. The resulting gallium nitride sintered body had a lowstrength, and cracking occurred when the sintered body was taken out ofthe carbon-made die used in the hot-pressing. The density, openporosity, X-ray peak intensity ratio, oxygen content, andpresence/absence of cracking of the obtained sintered body are shown inTable 2.

First Embodiment—Comparative Example 3

A gallium nitride powder was obtained by subjecting a gallium oxidepowder to a nitridation treatment in the same manner as in Example 3,except that the nitridation temperature was changed to 1120° C. Aportion of the thus obtained gallium nitride powder was collected, andits specific surface area (BET) was measured. In addition, the untampedbulk density was measured in accordance with JIS Z2504, and the reposeangle was also measured using a powder tester (Model PT-N, manufacturedby Hosokawa Micron Group). The physical property values of the obtainedgallium nitride powder are shown in Table 1.

The thus obtained gallium nitride powder was subjected to ahot-pressing, nitridation, and bonding in the same manner as in Example3 to prepare a gallium nitride sintered body and a gallium nitridesputtering target. The thus obtained gallium nitride sintered body had alow strength, and cracking occurred when the sintered body was taken outof the carbon-made die used in the hot-pressing. The density, openporosity, X-ray peak intensity ratio, oxygen content, andpresence/absence of cracking of the sintered body are shown in Table 2.

TABLE 1 Nitridation temperature of gallium Surface Untamped oxide areabulk Repose powder (BET) density angle (° C.) (m²/g) (g/cm³) (°) FirstExample 1 — 14 0.551 39 embodiment Example 2 — 14 0.551 39 Example 31050 6 0.438 37 Example 4 1000 8 0.410 38 Example 5 1100 0.8 0.800 28Example 6 1050 6 0.438 37 Example 7 1050 6 0.438 37 Comparative — 140.551 39 Example 1 Comparative 960 12 0.380 41 Example 2 Comparative1120 0.3 1.000 25 Example 3

TABLE 2 Nitridation X-ray Presence/ Sintering Sintered Open temperatureof intensity Oxygen absence of Sintering temperature Pressure densityporosity sintered body ratio content breakage/ method (° C.) (MPa)(g/cm³) (%) (° C.) (%) (atm %) cracking First Example 1 HP 1000 40 2.7598 1000 0.0 4.5 absent Embodiment Example 2 HP 1000 100 4.05 88  900 0.04.9 absent Example 3 HP 1050 50 3.04 76 1050 0.0 3.3 absent Example 4 HP1050 50 2.80 80 1050 0.0 3.8 absent Example 5 HP 1050 50 3.35 72 10500.0 1.3 absent Example 6 HP 1050 100 4.14 80 1050 0.0 5.1 absent Example7 HIP 1050 260 4.80 70 1050 2.5 9.6 absent Comparative HP 1000 40 2.7098 — 5.9 15.2 absent Example 1 Comparative HP 1050 50 2.43 90 1050 4.713.2 present Example 2 Comparative HP 1050 50 4.26 60 1050 0.0 0.9present Example 3

The second embodiment of the present invention will now be described byway of examples thereof; however, this embodiment is not limitedthereto.

(Measurement of Repose Angle and Untamped Bulk Density of GalliumNitride Powder)

The repose angle of a gallium nitride powder, which is a parameter ofthe fluidity, was measured using a powder tester (Model PT-N,manufactured by Hosokawa Micron Group). The untamped bulk density of agallium nitride powder was measured in accordance with JIS Z2504.

The repose angle and untamped bulk density of the gallium nitridepowders used in the respective examples are as shown below.

TABLE 3 Bulk Repose density angle (g/cm³) (°) Second Example 1 0.3 46embodiment Example 2 1.2 34 Example 3 1.2 34 Example 4 1.2 34 Example 50.3 46 Example 6 0.3 46 Comparative Example 1 0.3 46 Comparative Example2 0.3 46 Comparative Example 3 0.3 46(Measurement of Density of Gallium Nitride Molded Article)

The density of a gallium nitride molded article obtained from therespective gallium nitride powders was calculated, based on its weightand the volume which is estimated from the apparent shape of the moldedarticle.

(Measurement of Resistivity of Molded Article)

The resistivity of a molded article having low resistance was measuredin accordance with a 4-probe method by use of Loresta HPMCP-T410 andthat of a molded article having high resistance was measured by use ofHiresta MCP-T450.

The obtained metal gallium-impregnated gallium nitride molded articlewas bonded onto a backing plate made of Cu by using an In solder as abonding material to obtain a gallium nitride-based sputtering target.

A film was formed by sputtering under the following film formationconditions by use of the obtained target, and the thus obtained film wasevaluated:

Electric discharge method: RF sputtering or DC sputtering Film formationapparatus: Magnetron sputtering apparatus (manufactured by TokudaSeisakusho Co., Ltd.; CFS-4ES for 76.2 mmϕ or CFS-8EP for 127 mmϕ)

Target size: 76.2 mmϕ or 127 mmϕ

Film formation pressure: 0.8 Pa

Added gas: Nitrogen

Electric discharge power: 100 W.

Second Embodiment—Example 1

A gallium nitride powder in an amount of 100 g (purity: 4 N,manufactured by Kojundo Chemical Lab. Co., Ltd.) was loaded in a 102 mmϕcarbon-made die to perform hot-pressing. The hot-pressing treatment wasperformed by heating the gallium nitride powder at a rate of 200° C./hto a final temperature of 1000° C., increasing the pressing pressure to40 MPa when the temperature reaches 1000° C., and maintaining thetemperature and pressure for 2 hours. After the 2 hours of retentiontime, the resultant was cooled to about 50° C. over a period of 5 hoursand the die was taken out to remove a gallium nitride sintered body. Thethus obtained sintered body had a size of about 100 mmϕ and density of2.69 g/cm³. The sintered body was then processed into a 76.2 mmϕ×2 mmtdisk.

Then, 24.5 g of the thus processed gallium nitride sintered body and33.0 g of metal gallium (purity: 6 N, oxygen content: 0.0174 atm %;manufactured by Dowa Electronics Materials CO., Ltd.) were placed in avacuum packaging bag such that the metal gallium was arranged in theperiphery of the gallium nitride sintered body. The vacuum packaging bagin which the gallium nitride sintered body and the metal gallium wereplaced was then vacuumed under reduced pressure of 1000 Pa tovacuum-package the gallium nitride sintered body along with the metalgallium. The thus packaged container was then heated to about 50° C. tocompletely melt the metal gallium and the resultant was then subjectedto a cold isostatic pressing (CIP) at 100 MPa for 60 seconds to obtain amolded article. After removing the molded article from the CIP, themolded article was heated at about 50° C. to remove the metal galliumremaining in the periphery to obtain a metal gallium-impregnated galliumnitride molded article. The thus obtained metal gallium-impregnatedgallium nitride molded article had density of 5.26 g/cm³ and resistanceof 4.30×10⁻³ Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an electron probe microanalyzer (EPMA;EPMA1610, manufactured by Shimadzu Corporation), and there wereconfirmed spots where nitrogen and gallium were in coexistence and spotswhere nitrogen was not detected excluding the background and gallium waspredominantly present. In addition, a scanning electron microscope (SEM;JSM-7600F, manufactured by JEOL Ltd.) observation was performed for thesame cross-section of the molded article to verify the positions ofvoids in the cross-section, and it was confirmed that the value ofGa/(Ga+N) in the cross-section of the molded article was 69% in terms ofmolar ratio and that the volume ratio of metal gallium with respect tothe volume of the voids in the gallium nitride sintered body was 78%.Moreover, the same cross-section of the molded article was analyzed by apowder X-ray diffraction (XRD; RINT Ultima III, manufactured by RigakuCorporation), and it was confirmed that gallium nitride and metalgallium coexisted as shown in the X-ray diffraction spectrum shown inFIG. 7.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target,and a film having no cracking or the like was obtained by both RFsputtering and DC sputtering; therefore, it was confirmed that a filmcan be formed by both RF sputtering and DC sputtering by using thegallium nitride-based sputtering target of this Example. Further, therate of film formation by DC sputtering was found to be 35 nm/min;therefore, it was confirmed that a film can be formed at a high rate.

Second Embodiment—Example 2

50 g of the same gallium nitride powder as used in Example 1 and 1000 gof iron-cored resin balls of 15 mm in diameter were loaded in a 1-Lnylon pot. The loaded materials were dry-mixed for 20 hours using arotary ball mill, and the resin balls and coarse particles were thenremoved using a 500-μm sieve. Thereafter, the thus sieved powder wassubjected to rolling granulation for 2 hours and the resultant wascollected to obtain a gallium nitride granulated powder in an amount ofabout 50 g. A gallium nitride granulated powder was prepared twice bythis method to obtain a total of 100 g of a gallium nitride granulatedpowder.

The entire amount of the obtained gallium nitride granulated powder wasloaded in a 102 mmϕ carbon-made die to perform hot-pressing. Thehot-pressing treatment was performed by heating the gallium nitridegranulated powder at a rate of 200° C./h to a final temperature of 1000°C., increasing the pressing pressure to 40 MPa when the temperaturereaches 1000° C., and maintaining the temperature and pressure for 2hours. After the 2 hours of retention time, the resultant was cooled toabout 50° C. over a period of 5 hours and the die was taken out toremove a gallium nitride sintered body. The thus obtained sintered bodyhad a size of about 100 mmϕ and density of 3.16 g/cm³. The sintered bodywas then processed into a 76.2 mmϕ×2 mmt disk.

Then, 28.8 g of the thus processed gallium nitride sintered body and 30g of the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride sintered body. The vacuum packaging bag in whichthe gallium nitride sintered body and the metal gallium were placed wasthen vacuumed under reduced pressure of 1000 Pa to vacuum-package thegallium nitride sintered body along with the metal gallium. The thuspackaged container was then heated to about 50° C. to completely meltthe metal gallium and the resultant was then subjected to a coldisostatic pressing (CIP) at 100 MPa for 60 seconds to obtain a moldedarticle. After removing the molded article from CIP, the molded articlewas heated at about 50° C. to remove the metal gallium remaining in theperiphery to obtain a metal gallium-impregnated gallium nitride moldedarticle. The thus obtained metal gallium-impregnated gallium nitridemolded article had density of 5.30 g/cm³ and resistance of 8.60×10⁻²Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and, there were confirmedspots where nitrogen and gallium were in coexistence and spots wherenitrogen was not detected excluding the background and gallium waspredominantly present. In addition, a scanning electron microscopicimage (SEM image) was examined for the same cross-section of the moldedarticle to verify the positions of voids in the cross-section, and itwas confirmed that the value of Ga/(Ga+N) in the cross-section of themolded article was 65% in terms of molar ratio and that the volume ratioof metal gallium with respect to the volume of the voids in the galliumnitride sintered body was 75%.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target,and a film having no particular cracking or the like was obtained byboth RF sputtering and DC sputtering; therefore, it was confirmed that afilm can be formed by both RF sputtering and DC sputtering by using thegallium nitride-based sputtering target of this Example. Further, therate of film formation by DC sputtering was found to be 35 nm/min;therefore, it was confirmed that a film can be formed at a high rate.

Second Embodiment—Example 3

A total of 350 g of a gallium nitride granulated powder was prepared inthe same manner as in Example 2 and the entire amount thereof was loadedin a 170-mmϕ carbon-made die to perform hot-pressing. The hot-pressingtreatment was performed by heating the gallium nitride granulated powderat a rate of 200° C./h to a final temperature of 1000° C., increasingthe pressing pressure to 40 MPa when the temperature reaches 1000° C.,and maintaining the temperature and pressure for 2 hours. After the 2hours of retention time, the resultant was cooled to about 50° C. over aperiod of 5 hours and the die was taken out to remove a gallium nitridesintered body. The thus obtained sintered body had a size of about 170mmϕ and density of 3.09 g/cm³. The sintered body was then processed intoa 127 mmϕ×3 mmt disk.

Then, 117.5 g of the thus processed gallium nitride sintered body and120 g of the same metal gallium as used in Example 1 were placed in avacuum packaging bag such that the metal gallium was arranged in theperiphery of the gallium nitride sintered body. The vacuum packaging bagin which the gallium nitride sintered body and the metal gallium wereplaced was then vacuumed under reduced pressure of 10 Pa tovacuum-package the gallium nitride sintered body along with the metalgallium. The thus packaged container was then heated to about 50° C. tocompletely melt the metal gallium and the resultant was then subjectedto a cold isostatic pressing (CIP) at 100 MPa for 60 seconds to obtain amolded article. After removing the molded article from CIP, the moldedarticle was heated at about 50° C. to remove the metal gallium remainingin the periphery to obtain a metal gallium-impregnated gallium nitridemolded article. The thus obtained metal gallium-impregnated galliumnitride molded article had density of 5.23 g/cm³ and resistance of6.20×10⁻³ Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent. In addition, a SEM observation was performed for the samecross-section of the molded article to verify the positions of voids inthe cross-section, and it was confirmed that the value of Ga/(Ga+N) inthe cross-section of the molded article was 65% in terms of molar ratioand that the volume ratio of metal gallium with respect to the volume ofthe voids in the gallium nitride sintered body was 73%.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target,and a film having no particular cracking or the like was obtained byboth RF sputtering and DC sputtering; therefore, it was confirmed that afilm can be formed by both RF sputtering and DC sputtering by using thegallium nitride-based sputtering target of this Example, and that alarge-sized sputtering target can be produced.

Second Embodiment—Example 4

A gallium nitride sintered body was prepared and processed in the samemanner as in Example 2 to obtain a sintered body in the form of a 76.2mmϕ×2 mmt disk. The obtained sintered body had density of 3.10 g/cm³.

Then, 29 g of the thus processed gallium nitride sintered body and 30 gof the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride sintered body. The vacuum packaging bag in whichthe gallium nitride sintered body and the metal gallium were placed wasthen vacuumed under reduced pressure of 1000 Pa to vacuum-package thegallium nitride sintered body along with the metal gallium. The thuspackaged container was then heated to about 50° C. to completely meltthe metal gallium and the resultant was then subjected to the CIP at 100MPa for 60 seconds to obtain a molded article. After removing the moldedarticle from the CIP, the molded article was heated at about 50° C. toremove the metal gallium remaining in the periphery to obtain a metalgallium-impregnated gallium nitride molded article. The thus obtainedmetal gallium-impregnated gallium nitride molded article washeat-treated in vacuum at 200° C. for 2 hours and then cooled to roomtemperature. The resulting metal gallium-impregnated gallium nitridemolded article had density of 4.60 g/cm³ and resistance of 5.1×10⁻³Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent. In addition, a SEM observation was performed for the samecross-section of the molded article to verify the positions of voids inthe cross-section, and it was confirmed that the value of Ga/(Ga+N) inthe cross-section of the molded article was 62% in terms of molar ratioand that the volume ratio of metal gallium with respect to the volume ofthe voids in the gallium nitride sintered body was 51%.

The thus obtained metal gallium-impregnated gallium nitride sinteredbody was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target,and a film having no particular cracking or the like was obtained byboth RF sputtering and DC sputtering; therefore, it was confirmed that afilm can be formed by both RF sputtering and DC sputtering by using thegallium nitride-based sputtering target of this Example.

Second Embodiment—Example 5

The same gallium nitride powder as used in Example 1 (100 g) was loadedin a 102-mmϕ carbon-made die to be press-molded at room temperature witha pressure of 30 MPa. Thereafter, the resultant was subjected to a CIPtreatment at 300 MPa to obtain a gallium nitride molded article havingdensity of 2.19 g/cm³. The obtained molded article was then processedinto a 76.2 mmϕ×2 mmt disk.

Then, 20.0 g of the thus processed gallium nitride molded article and 38g of the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride molded article. The vacuum packaging bag in whichthe gallium nitride molded article and the metal gallium were placed wasthen vacuumed under reduced pressure of 300 Pa to vacuum-package thegallium nitride molded article along with the metal gallium. The thuspackaged container was then heated to about 50° C. to completely meltthe metal gallium and the resultant was then subjected to a CIPtreatment at 100 MPa for 60 seconds to obtain a molded article. Afterremoving the molded article from the CIP, the molded article was heatedat about 50° C. to remove the metal gallium remaining in the peripheryto obtain a metal gallium-impregnated gallium nitride molded article.The thus obtained metal gallium-impregnated gallium nitride moldedarticle had density of 5.48 g/cm³ and resistance of 2.40×10⁻³ Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent. In addition, a SEM observation was performed for the samecross-section of the molded article to verify the positions of voids inthe cross-section, and it was confirmed that the value of Ga/(Ga+N) inthe cross-section of the molded article was 74% in terms of molar ratioand that the volume ratio of metal gallium with respect to the volume ofthe voids in the gallium nitride sintered body was 87%.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target,and a film having no particular cracking or the like was obtained byboth RF sputtering and DC sputtering; therefore, it was confirmed that afilm can be formed by both RF sputtering and DC sputtering by using thegallium nitride-based sputtering target of this Example.

Second Embodiment—Example 6

The same gallium nitride powder as used in Example 1 (100 g) was loadedin a 102 mmϕ carbon-made die to perform hot-pressing. The hot-pressingtreatment was performed by heating the gallium nitride powder at a rateof 200° C./h to a final temperature of 1050° C., increasing the pressingpressure to 50 MPa when the temperature reaches 1050° C., andmaintaining the temperature and pressure for 2 hours. After the 2 hoursof retention time, the resultant was cooled to about 50° C. over aperiod of 5 hours and the die was taken out to remove a gallium nitridesintered body. The thus obtained sintered body had density of 3.04g/cm³. The sintered body was then processed into a 76.2 mmϕ×2 mmt disk.

Then, 27.7 g of the thus processed gallium nitride sintered body and 38g of the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride sintered body. The vacuum packaging bag in whichthe gallium nitride sintered body and the metal gallium were placed wasthen vacuumed under reduced pressure of 300 Pa to vacuum-package thegallium nitride sintered body along with the metal gallium. The thuspackaged container was then heated to about 50° C. to completely meltthe metal gallium and the resultant was then subjected to a CIPtreatment at 100 MPa for 60 seconds to obtain a molded article. Afterremoving the molded article from the CIP, the molded article was heatedat about 50° C. to remove the metal gallium remaining in the peripheryto obtain a metal gallium-impregnated gallium nitride molded article.The thus obtained metal gallium-impregnated gallium nitride moldedarticle had density of 5.32 g/cm³ and resistance of 1.80×10⁻³ Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent. In addition, a SEM observation was performed for the samecross-section of the molded article to verify the positions of voids inthe cross-section, and it was confirmed that the value of Ga/(Ga+N) inthe cross-section of the molded article was 72% in terms of molar ratioand that the volume ratio of metal gallium with respect to the volume ofthe voids in the gallium nitride sintered body was 81%.

A portion of the thus obtained metal gallium-impregnated gallium nitridemolded article was collected and its oxygen content was measured usingan oxygen-nitrogen analyzer (manufactured by LECO Corporation), and itwas found that the metal gallium-impregnated gallium nitride moldedarticle had an oxygen content of 6.03 atm %. In addition, the samecross-section of the molded article was analyzed by a powder X-raydiffraction (XRD; RINT Ultima III, manufactured by Rigaku Corporation),and the X-ray diffraction spectrum comparable to the one observed inExample 1 was obtained; therefore, it was confirmed that the galliumnitride and the metal gallium coexisted. Moreover, it was found that theobtained metal gallium-impregnated gallium nitride molded articlecontained either no gallium oxide or only a trace amount of galliumoxide below the lower detection limit, since there was no (002) peak ofgallium oxide in the analyzed X-ray diffraction spectrum.

Furthermore, the metal gallium-impregnated gallium nitride moldedarticle had thermal conductivity of 14.3 W/mK; therefore, it wasconfirmed that the thermal conductivity was improved as compared to thegallium nitride sintered body prior to being impregnated with metalgallium.

Then, RF sputtering was performed on a surface to be bonded (a surfaceto form an interface with a solder material) of the obtained metalgallium-impregnated gallium nitride molded article by using a tungstentarget to form a barrier layer of tungsten. This film formation wascarried out by a magnetron sputtering apparatus using the tungstentarget of 76.2 mmϕ in size, an added gas of argon, and an electricaldischarge powder of 100 W. The thus obtained tungsten barrier layer hada thickness of 2 μm.

The thus obtained gallium-impregnated gallium nitride molded articlehaving a barrier layer of tungsten formed thereon was bonded onto abacking plate made of Cu by using an indium solder as a bonding materialto obtain a gallium nitride-based sputtering target. Sputtering was thenperformed under the above-described film formation conditions by use ofthe obtained target, and a film having no particular cracking or thelike was obtained by both RF sputtering and DC sputtering; therefore, itwas confirmed that a film can be formed by both RF sputtering and DCsputtering by using the gallium nitride-based sputtering target of thisExample.

A portion of each of the metal gallium-impregnated gallium nitridemolded articles obtained in Examples 1 to 5 was collected to measure theoxygen content and the thermal conductivity in the same procedures. Themeasurement results are shown in Table 7.

Second Embodiment—Comparative Example 1

The same gallium nitride powder as used in Example 1 (100 g) was loadedin a 102 mmϕ carbon-made die to be press-molded at room temperature witha pressure of 30 MPa. Thereafter, the resultant was subjected to a CIPtreatment at 300 MPa to obtain a gallium nitride molded article havingdensity of 2.30 g/cm³. The obtained molded article was then processedinto a 76.2 mmϕ×2 mmt disk. The molded article had resistance of 2.6×10⁷Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence; however, a spot wherenitrogen was not detected excluding the background and gallium waspredominantly present was not observed. In addition, a SEM observationwas performed for the same cross-section of the molded article to verifythe positions of voids in the cross-section, and it was confirmed thatthe value of Ga/(Ga+N) in the cross-section of the molded article was50% in terms of molar ratio and that the volume ratio of metal galliumwith respect to the volume of the voids in the gallium nitride sinteredbody was 0%.

The thus obtained molded article was bonded onto a backing plate made ofCu by using an indium solder as a bonding material to obtain a galliumnitride-based sputtering target. When sputtering was performed using theobtained target under the same film formation conditions as in Example1, cracking occurred in the target, and a film could not properlyformed.

Second Embodiment—Comparative Example 2

A gallium nitride molded article was produced in the same manner as inExample 5. The obtained gallium nitride molded article had density of2.20 g/cm³.

Then, 20 g of the thus processed gallium nitride molded article and 4 gof the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride molded article. The vacuum packaging bag in whichthe gallium nitride molded article and the metal gallium were placed wasthen vacuumed under reduced pressure of 1000 Pa to vacuum-package thegallium nitride molded article along with the metal gallium. The thuspackaged container was then heated to about 50° C. to completely meltthe metal gallium, and the resultant was then subjected to a CIPtreatment at 100 MPa for 60 seconds to obtain a molded article. The thusobtained metal gallium-impregnated gallium nitride molded article haddensity of 2.58 g/cm³ and resistance of 2.00×10² Ω·cm.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent; however, the number of the spots where gallium waspredominantly present was smaller as compared to Example 5. In addition,spots where metal gallium did not impregnate were also confirmed byvisual observation of the molded article. Moreover, a SEM observationwas performed for the same cross-section of the molded article to verifythe positions of voids in the cross-section, and it was confirmed thatthe value of Ga/(Ga+N) in the cross-section of the molded article was54% in terms of molar ratio and that the volume ratio of metal galliumwith respect to the volume of the voids in the gallium nitride sinteredbody was 10%.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target.RF sputtering was able to be performed; however, DC sputtering could notbe performed.

Second Embodiment—Comparative Example 3

The same gallium nitride powder as used in Example 1 (100 g) wassintered by a hot isostatic pressing method using a 102 mmϕ die. The hotisostatic pressing treatment was performed by heating the galliumnitride powder at a rate of 100° C./h to a final temperature of 1050°C., increasing the pressing pressure to 280 MPa when the temperaturereaches 1050° C., and maintaining the temperature and pressure for 2hours. After the 2 hours of retention time, the resultant was cooled toabout 50° C. over a period of 10 hours and the die was taken out toremove a gallium nitride sintered body. The thus obtained sintered bodyhad density of 5.07 g/cm³ and resistance of 1.2×10⁷ Ω·cm. The sinteredbody was then processed into a 76.2 mmϕ2 mmt disk.

Then, 46.2 g of the thus processed gallium nitride sintered body and 9 gof the same metal gallium as used in Example 1 were placed in a vacuumpackaging bag such that the metal gallium was arranged in the peripheryof the gallium nitride sintered body. The vacuum packaging bag in whichthe gallium nitride sintered body and the metal gallium were placed wasthen vacuumed under reduced pressure of 1000 Pa to vacuum-package thegallium nitride sintered body along with the metal gallium. The thuspackaged container was heated to about 50° C. to completely melt themetal gallium and the resultant was then subjected to cold isostaticpressing (CIP) at 100 MPa for 60 seconds to obtain a molded article, inthe same manner as in Example 1. The thus obtained metalgallium-impregnated gallium nitride molded article had density of 5.34g/cm³ and resistance of 2.13×10² Ω·cm. It is noted here that, althoughthe amount of the metal gallium used was smaller than the weight of theprocessed gallium nitride sintered body, the amount of the metal galliumwas sufficient for the volume of the voids calculated from the volumeand density of the gallium nitride sintered body prior to beingimpregnated with the metal gallium, as in the metal gallium impregnationtreatment performed in Examples 1 to 6.

The obtained molded article was mirror-polished to expose across-section and the distribution of gallium and nitrogen in thecross-section was examined using an EPMA, and there were confirmed spotswhere nitrogen and gallium were in coexistence and spots where nitrogenwas not detected excluding the background and gallium was predominantlypresent; however, the number of the spots where gallium waspredominantly present was smaller as compared to Example 1. In addition,spots where metal gallium did not impregnate were also confirmed byvisual observation of the molded article. Moreover, it was confirmedthat the value of Ga/(Ga+N) in the cross-section was 53% in terms ofmolar ratio and that the volume ratio of metal gallium with respect tothe volume of the voids in the gallium nitride sintered body was 16%, byperforming a SEM observation of the same cross-section of the moldedarticle.

The thus obtained metal gallium-impregnated gallium nitride moldedarticle was bonded onto a backing plate made of Cu by using an indiumsolder as a bonding material to obtain a gallium nitride-basedsputtering target. Sputtering was then performed under theabove-described film formation conditions by use of the obtained target.RF sputtering was able to be performed; however, DC sputtering could notbe performed.

The processing conditions used in the respective Examples andComparative Examples of the second embodiment are shown in Table 4.

TABLE 4 Metal gallium impregnation step Gallium nitride molding stepAmount of Molding gallium nitride Amount of Vacuum CIP Moldingtemperature Pressure sintered body metal gallium pressure Pressuremethod (° C.) (MPa) (g) (g) (Pa) (MPa) Second Example 1 HP 1000 40 24.533 1000 100 Embodiment Example 2 HP 1000 40 28.8 30 1000 100 Example 3HP 1000 40 117.5 120 10 100 Example 4 HP 1000 40 29 30 1000 100 Example5 Uniaxial pressing 25 30 20 38 300 100 Example 6 HP 1050 50 27.7 38 300100 Comparative Uniaxial pressing 25 30 — — — — Example 1 ComparativeUniaxial pressing 25 30 20 4 1000 100 Example 2 Comparative HIP 1050 28046.2 9 1000 100 Example 3

Table 5 shows the density of the respective gallium nitride moldedarticles as well as the density, Ga/(Ga+N) in terms of molar ratio (%),the volume ratio of metal gallium contained in the voids, and theresistivity of the respective metal gallium-impregnated gallium nitridemolded articles obtained in Examples and Comparative Examples of thesecond embodiment.

TABLE 5 Density of metal Resistivity of metal Density ofgallium-impregnated Ratio of gallium-impregnated gallium nitride galliumnitride metal gallium gallium nitride molded article molded articleGa/(Ga + N) contained in voids molded article (g/cm³) (g/cm³) (%) (%) (Ω· cm) Second Example 1 2.69 5.26 69 78 4.30 × 10⁻³ Embodiment Example 23.16 5.30 65 75 8.60 × 10⁻² Example 3 3.09 5.23 65 73 6.20 × 10⁻³Example 4 3.10 4.60 62 51 5.10 × 10⁻³ Example 5 2.19 5.48 74 87 2.40 ×10⁻³ Example 6 3.04 5.32 72 81 1.80 × 10⁻³ Comparative 2.30 2.30 50 02.60 × 10⁷  Example 1 Comparative 2.20 2.58 54 10 2.00 × 10²  Example 2Comparative 5.07 5.34 53 16 2.13 × 10²  Example 3

Table 6 shows whether or not RF sputtering and DC sputtering could becarried out using the respective gallium nitride-based sputteringtargets that were obtained in Examples and Comparative Examples of thesecond embodiment.

TABLE 6 RF DC sputtering sputtering Second Example 1 Good Goodembodiment Example 2 Good Good Example 3 Good Good Example 4 Good GoodExample 5 Good Good Example 6 Good Good Comparative No Good No GoodExample 1 Comparative Good No Good Example 2 Comparative Good No GoodExample 3

Table 7 shows the density of the respective gallium nitride moldedarticles as well as the density, the oxygen content, X-ray intensityratio, and the thermal conductivity of the respective metalgallium-impregnated gallium nitride molded articles obtained in Examplesand Comparative Examples of the second embodiment.

TABLE 7 Density of metal Physical property values Density ofgallium-impregnated X-ray gallium nitride gallium nitride Oxygenintensity Thermal molded article molded article content ratioconductivity (g/cm³) (g/cm³) (atm %) (%) (W/mK) Second Example 1 2.695.26 8.39 0.0 13.9 embodiment Example 2 3.16 5.30 10.5 2.5 13.5 Example3 3.09 5.23 8.74 0.0 12.2 Example 4 3.10 4.60 9.22 0.7 11.5 Example 52.19 5.48 7.81 0.0 14.7 Example 6 3.04 5.48 6.03 0.0 14.3 Comparative2.30 — 16.1 8.5 2.6 Example 1 Comparative 2.20 2.58 12.5 3.4 2.8 Example2 Comparative 5.07 5.34 13.3 4.1 2.7 Example 3

DESCRIPTION OF SYMBOLS

-   1: Peak of the (002) plane of gallium nitride-   2: Peak of the (002) plane of gallium oxide-   11: Metal gallium-impregnated gallium nitride molded article-   12: Gallium nitride phase-   13: Metal gallium phase-   14: Void-   15: Metal gallium-   16: Vacuum packaging bag

The invention claimed is:
 1. A metal gallium-impregnated gallium nitridemolded article, wherein the molded article comprises a gallium nitridephase having voids contained therein and a metal gallium phase whichexist as separate phases, and the molded article has a molar ratio ofGa/(Ga+N) of 55% to 80%, wherein said gallium nitride phase has adensity of 2.5 g/cm³ to less than 5.0 g/cm³ and a composition having anintensity ratio of a gallium oxide peak of the (002) plane to a galliumnitride peak of the (002) plane of less than 3%, as determined by X-raydiffraction analysis.
 2. The metal gallium-impregnated gallium nitridemolded article according to claim 1, wherein not less than 30% of atotal volume of said voids contained therein is filled with said metalgallium.
 3. The metal gallium-impregnated gallium nitride molded articleaccording to claim 1, wherein the molded article has density of 3.20g/cm³ to less than 6.05 g/cm³.
 4. The metal gallium-impregnated galliumnitride molded article according to claim 1, wherein the molded articlehas resistance of not higher than 1 Ω·cm.
 5. The metalgallium-impregnated gallium nitride molded article according to claim 1,wherein the molded article contains oxygen in an amount of not more than11 atm %.
 6. The metal gallium-impregnated gallium nitride moldedarticle according to claim 1, wherein said voids contained thereincomprise open pores and closed pores, and the volume ratio of said openpores with respect to a total volume of said voids is not less than 70%.7. A method of producing the metal gallium-impregnated gallium nitridemolded article according to claim 1, wherein the method comprisesimpregnating a liquid metal gallium into a gallium nitride moldedarticle having density of 2.0 g/cm³ to less than 5.0 g/cm³.
 8. Themethod according to claim 7, wherein said impregnating the liquid metalgallium into a gallium nitride molded article includes subjecting saidgallium nitride molded article and said metal gallium to a vacuumtreatment in the same container and then isotropically applying apressure to said container.
 9. The method according to claim 7, whereinsaid gallium nitride molded article is obtained by a process comprising:obtaining a molded article by sintering a gallium nitride powder havinga specific surface area (BET) of 0.4 m²/g to 15 m²/g, untamped bulkdensity of not less than 0.4 g/cm³, and a repose angle of not largerthan 40°; and heat-treating the obtained molded article in anammonia-containing atmosphere.
 10. The method according to claim 9,wherein said gallium nitride powder is obtained by subjecting a galliumoxide powder to a nitridation treatment in an ammonia atmosphere at atemperature of 1000° C. to 1100° C.
 11. A gallium nitride sputteringtarget, comprising the metal gallium-impregnated gallium nitride moldedarticle according to claim 1.